The present invention is related to polarization maintaining fibers, and more specifically to packaged polarization maintaining fibers and methods of packaging polarization maintaining fibers.
Polarization maintaining fibers, also known as polarization preserving fibers, are used to transmit optical signals while maintaining an optical signal""s polarization. The polarization maintaining fibers typically include a fiber core surrounded by a cladding having a refractive index less than the refractive index of the fiber core. A polarization maintaining fiber works to maintain the polarization of a light signal by inducing birefringence within the fiber core. The birefringence produces two optical transmission axes within the fiber, commonly known as the fast and slow axes of the fiber. The axes are orthogonal to both each other and to the propagation direction of an optical signal.
The polarization maintaining fibers typically have substantially circular cross-sections. Birefringence may be created within a polarization maintaining fiber by inducing constant stresses within the fiber with stress applying parts (stress induced birefringence). Such fibers are commonly referred to as SAP fibers. Birefringence may also be created within a polarization maintaining fiber by forming a non-circular fiber core (shape induced birefringence). The four main types of polarization maintaining fibers are the bowtie fiber, the panda fiber, the elliptical jacket fiber, and the elliptical core fiber. The slow axis of these polarization maintaining fiber exhibits a higher stress at a cladding-solder interface than the fast axis.
The quality of an optical fiber in maintaining the polarization of an optical signal may be judged by the optical fiber""s extinction ratio. The larger the extinction ratio, the less an optical signal de-couples itself as it propagates along an axis of the polarization maintaining fiber. External stresses applied to a polarization maintaining fiber can degrade this extinction ratio. These stresses can occur from the packaging surrounding the optical fiber cladding. Polarization maintaining fibers are often packaged in order to help align and fix the fiber in a secure and stable manner with respect to other devices, such as a lasers.
During packaging, the body portion of the polarization maintaining fiber is typically surrounded with a solder preform and the solder preform is surrounded with a sleeve. The xe2x80x9cbody portionxe2x80x9d of the polarization maintaining fiber is the longitudinally extending section of the polarization maintaining fiber connecting two distal ends of the polarization maintaining fiber. The solder is then melted and allowed to solidify to secure the optical fiber within the sleeve. The packaging, however, causes stresses to occur at the interface of the cladding of the polarization maintaining fiber and the solder. These stresses may cause breakages in the optical fiber core or cladding. Further, degradation of the extinction ratio may occur because, although it is currently possible to align the slow and fast axes in a predetermined manner during fabrication, it is not currently possible to precisely position a polarization maintaining fiber at the center of the packaging, i.e., the polarization maintaining fiber is typically eccentric. If a polarization maintaining fiber is positioned at the center of the packaging, the stresses on the cladding from the packaging are equal, and the packaging induced stresses have no effect on the fiberes extinction ratio, although the polarization maintaining fiber is still subject to stress induced breakage. However, a designer can not predict where a polarization maintaining fiber is ultimately positioned within a circular packaging after the solder solidifies, thereby making it impossible to align the slow and fast axes in a predetermined manner to better protect against stress induced degradation of the extinction ratio.
Therefore, there remains a need to reduce or remove the stresses occurring at the solder-cladding interface of a packaged polarization maintaining fiber. Still further, there remains a need to better orient the slow and fast optical axes of a polarization maintaining fiber within the packaging in order to better protect the polarization maintaining fiber against stress induced degradation of its extinction ratio.
The present invention is a packaged polarization maintaining fiber and method of packaging polarization maintaining fiber. A packaged polarization maintaining fiber according to the present invention includes a cylindrical polarization maintaining fiber and a sleeve surrounding a body portion of said polarization maintaining fiber. The polarization maintaining fiber is secured within the sleeve by solder. The sleeve and solder have a substantially non-circular symmetrical cross-section defined by a long axis and a short axis, and a selected one of the long or short axes has a higher stress than the other axis at an interface between the polarization maintaining fiber and the solder. The slow optical axis of the polarization maintaining fiber is substantially aligned with the selected axis and the fast optical axis of the polarization maintaining fiber is thereby substantially aligned with the other axis of the substantially non-circular symmetrical cross-section. The present invention thereby allows for a slow and fast optical axis of the polarization maintaining fiber to be oriented within the packaging to better preserve the extinction ration of the polarization maintaining fiber against stresses resulting from eccentricity. The packaging also helps reduce the magnitude of stresses induced on the fiber through eccentricity, as well as provide for higher yields during manufacturing of packaged polarization maintaining fibers.
In another embodiment of the present invention, a packaged polarization maintaining fiber includes a cylindrical polarization maintaining fiber having a circular cross-section and a sleeve surrounding the body portion of the fiber. The polarization maintaining fiber is secured within the sleeve with solder. The sleeve and solder have a substantially circular cross-section. The sleeve is comprised of a sleeve material and the solder is comprised of a solder material. The sleeve and solder material and the distance from the center of the polarization maintaining fiber to an interface between the solder and the sleeve and the distance from the center of the polarization maintaining fiber to an outer edge of the sleeve are selected such that the stress at and interface between the polarization maintaining fiber and the solder is approximately zero. This embodiment of the present invention allows for material and size selections to reduce stresses exerted on the polarization maintaining fiber, thereby reducing packaging induced stress-related breakages in the fiber and cladding surrounding the fiber. Further, lower stresses also help to preserve the extinction ratio of the polarization maintaining fiber should there be any imperfections in the packaging, such as through eccentricity. Higher yields in manufacturing polarization maintaining fibers are thereby accomplished.
The above and other features of the present invention will be better understood from the following detailed description of the preferred embodiments of the invention that is provided in connection with the accompanying drawings.